The exhaust air from a granulation plant for producing a urea-containing granulate contains urea-containing dusts and ammonia and purifying of the exhaust air is therefore necessary before it can be discharged into the atmosphere. Known processes for exhaust air scrubbing in ammonia-containing gases generate an ammonium sulfate solution having a low pH (about pH 2-4) which cannot be directly subjected to further processing in this acidic form. An upward pH adjustment is therefore necessary to avoid corrosion damage in downstream process stages.
EP 2 477 961 B1 discloses a process for recovering urea dust and ammonia from a gas stream, wherein the gas stream is contacted with a sulfuric acid solution to form an acid solution of ammonium sulfate and urea, wherein this acid solution is admixed with ammonia. In this known process a gas stream containing air, urea and ammonia is contacted with an aqueous sulfuric acid solution in a scrubbing apparatus to form an acid solution of urea and ammonium sulfate. This acid solution is subsequently neutralized in a neutralization apparatus by addition of ammonia and then concentrated in a further apparatus. In a mixing apparatus the ratio of ammonium sulfate to urea may subsequently be increased to a desired value by addition of ammonium sulfate before the concentrated melt is finally processed into a solid by granulation or spray crystallization to obtain a solids mixture of urea and ammonium sulfate as the end product.
EP 2 192 099 A1 discloses a urea granulation process containing an acidic exhaust air treatment. The ammonium sulfate formed in the acidic exhaust air treatment may be returned into the granulation process.
WO 2016/074813 A1 discloses a urea granulation process comprising an exhaust air treatment comprising a dust scrubbing and an acidic exhaust air scrubbing.
Thus a need exists for a process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate that dispenses with an external post-neutralization as a separate process step.